Preparation, properties and application of completely compensated Ge-on-GaAs films

We present and generalize the preparation conditions and properties of strongly compensated Ge films grown on semi-insulating GaAs(100) substrates by vacuum evaporation. The films are characterized using high-resolution X-ray diffraction (HRXD), atomic force (AFM) and Kelvin probe force microscopy (KPFM), Raman spectroscopy, electroreflectance, optical absorption and temperature-dependent transport measurements. HRXD shows that an ideally pseudomorphic growth of Ge films with good single crystal structure can be obtained by this growth technique. The film microstructure and transport properties are found to depend sensitively on the Ge deposition rate. Thin (~ 100 nm) Ge films grown at low rates exhibit rough, granular morphology (RMS up to 15 nm), high resistivity (up to 140 Ω·cm), low free charge carrier mobility (~ 50 cm²/V·s) and concentration (~ 10¹⁴–10¹⁵ cm⁻³) and activated conductivity with an anomalously large activation energy up to half the Ge bandgap. This insulating behavior is attributed to a completely compensated, disordered state arising from Ga and As impurity incorporation and large-scale potential fluctuations. Analysis suggests a two-dimensional percolative transport mechanism, consistent with the reduced film thickness and disorder-induced carrier localization. In contrast, films deposited at higher rates are smoother, more conductive, and exhibit weak temperature dependence of the resistivity, indicative of a more ordered state. These results provide insight into the interplay between disorder, carrier localization, and transport in heavily compensated semiconductors, and highlight the critical role of growth kinetics in determining the properties of Ge-on-GaAs heterostructures.